1. Field of the Invention
The present invention relates to a method and apparatus for controlling a continuously variable transmission during vehicle launch at low speeds to allow acceleration to a desired vehicle speed while providing a smooth ride.
2. Background Art
A continuously variable transmission (CVT) may be used to transfer torque from an engine to a vehicle's wheels. A CVT may comprise a drive sheave wheel and a driven sheave wheel that engage a drive belt or chain and are dynamically adjusted. The effective diameter of the drive sheave wheel and the driven sheave wheel may be varied according to a desired CVT ratio. The CVT ratio is the ratio of the drive sheave wheel effective diameter to the driven sheave wheel effective diameter. Rotation of the drive wheel is transferred to the driven wheel by the CVT belt or chain.
One of the benefits of a CVT is that the CVT may be controlled by a CVT controller to permit the engine to be operated to optimize fuel efficiency. The CVT permits the vehicle speed to vary even though the engine speed is held constant by simply changing the CVT ratio. The engine speed is not required to be constant. Generally, engine speed may be varied independently of the vehicle road speed.
When a vehicle is launched from a low speed or standing stop, the engine must accelerate from idle to the desired operating speed of the engine. The engine must overcome engine inertia as the engine accelerates. Part of the torque from the engine is used by the engine to overcome engine inertia. For example, as much as fifty percent of engine torque can be used at low speed conditions to accelerate the engine and overcome the engine's inertia. As a result, less torque is provided to the wheels of the vehicle during low speed operations as the engine is accelerated for the purpose of overcoming the engine's own inertia.
An undesirable lurching motion may occur when acceleration of the engine to the desired vehicle speed is completed. When the engine reaches its desired speed, there is no need to further accelerate the engine and the engine inertia load is reduced essentially to zero. All of the torque from the engine is then directed to the wheels of the vehicle. When torque to the wheels spikes, it causes the vehicle to lurch to such an extent it may be felt by a driver or passenger.
Another important characteristic when evaluating vehicle performance is vehicle acceleration from launch. It is generally desirable to accelerate from launch at maximum engine acceleration so that the engine may quickly run at its optimum speed/power. It is generally considered to be undesirable to reduce the acceleration of the engine because such a reduction in turn reduces acceleration of the vehicle.
The desired engine speed is ramped from the engine launch speed to the engine target speed generally at maximum acceleration. This may cause the CVT to begin variating before the target speed is reached. During engine acceleration, for example, 50% of the engine's torque may be used to overcome inertia, while 50% of the engine's torque may be provided to the vehicle wheels. When the engine inertia is overcome as the vehicle reaches its operating speed, the engine transitions to providing 90-95% of the engine's torque to the vehicle wheels.
The present invention is directed to solving the above problems and other problems according to the method and system as summarized below.